Unlimited disk space reseller

He has a fixed disk space and a fixed traffic, and unlimited clients, sites and domains.
It doesn't matter to me how many space he gives to her sites. I only need to know how many traffic and space he is *really* using.

I cannot imagine how to recreate this scenario with ISPconfig.

I tried to give him unlimited space, but if he gives unlimited space "-1" to any site, ispconfig tells me reseller is using "-1 MB"!!!

If he limits a site to 200 MB, ISPConfig tells me that he is using 200 MB (which is not true)

If I limit his space, he cannot give unlimited space to a site, nor put limits beyond assigned space (it doesn't necessary means that he is using more space than the limit!)

Is it possible to know how many space is *really* using a reseller instead of the space he has assigned?